1. Field of the Invention
The present invention relates to a controller for feeding chopped electricity, and more particularly to a controller for feeding chopped electricity to a load such as an electric motor.
2. Description of the Prior Art
In general, in the control of the level of the current flowing through a load such as a coil of an electric motor, the load is connected to a direct current electric source via a switching element such as a transistor and a value of the current flowing through the coil is detected by a resistor or the like. The detected current value is compared with a predefined reference current value in a binary fashion. When the reference current value is greater than the detected current value, the switching element is turned on. On the other hand, when the reference current value equals or falls below the detected current value, the switching element is turned off. Thereby, the switching element repeats ON and OFF, so that the average value of the current practically flowing through the load is controlled to a value corresponding to or close to the reference current value.
As disclosed in Japanese Patent Application Laid-Open Publication No. 8 (1996)-17S793, in the controller for feeding chopped electricity of this kind, the higher the ON/OFF frequency (chopping frequency) of the switching element, the more the energy loss increases in the switching element and other components, thus increasing calorific power. Therefore, it is not preferable to increase the chopping frequency so much. When the chopping frequency is too low, a difference between the reference current value and the practical current value increases, thus dropping the accuracy of the control. Further, it is not preferable for the chopping frequency to fall within the human audible frequency range because mechanical vibrations caused by the chopping of the current sounds as noise to human ears.
The controller for feeding chopped electricity is generally designed so that the chopping frequency is a value (e.g. 15 KHz) slightly higher than the upper limit of the human audible frequency range. However, in the case that the switching element is turned on or off by means of the binary signals obtained by the simple comparison between the reference current value and the practical current value, the chopping frequency changes and therefore it is not possible to expect a stable operation. Since the impedance of the coil changes with the change of the rotational angle of a rotor, the slope, of a rising curve of the current at the ON condition of the switching element and the slope of a descending curve of the current at the OFF condition of the switching element vary and therefore the chopping frequency varies. After all, the increase of the calorific power and the generation of the noise are caused with the variation of the impedance of the coil. Further, if there is scatter of the characteristics of the controller, abnormal calorification or abnormal noise is generated.
In the above mentioned Japanese Patent Application Laid-Open Publication No. 8 (1996)-17S793, two controllers for feeding chopped electricity which stabilize the chopping frequency are disclosed therein. In these controllers, a first signal corresponding to a current which is chopped and which actually flows through the load is compared with a second signal defining a reference current value and a binary signal corresponding to the result of the comparison is utilized for controlling ON/OFF energization of the load. According to one of these controllers, an ON/OFF signal which is supplied to an element for switching the load (a switching element) is turned to off, when the actual current value exceeds the reference current value. Further, whenever a period of the ON/OFF signal becomes a constant reference chopping timing (e.g. 15 kHz), the actual current value is compared with the reference current value. When the reference current value is greater than the actual current value, the ON/OFF signal is turned to on. On the contrary, when the reference current value equals to or falls below the actual current value, the ON/OFF signal is maintained in the off status. On the other hand, the other controller includes means for generating a reference chopping timing which is generated with approximately constant period, means for generating an enable flag signal which is set to a first level whenever the binary signal is turned from on status to off status and which is set to a second level whenever the period of the ON/OFF signal becomes the reference chopping timing and prohibition means for prohibiting the turning on from off status of the ON/OFF signal while the enable flag signal is in the first level.
When the controller for feeding chopped electricity disclosed in the above publication is applied for controlling a current which flows through coils of a switched reluctance motor, the impedance of the coils wound on poles of a stator is changed with a change of a rotational angle of & rotor. Thus the slope of a rising curve of the actual current value and the slope of a descending curve of the actual current with respect to the turning on or off of the current flowing through the coils are varied as shown in FIG. 3. In FIG. 3, a signal S1 is an actual current signal, a signal 53 is a reference current signal, a signal S6 is a reference chopping timing signal, a signal S9 is a signal which is generated by the result of the comparison between the signals S1 and S3 and a signal S12 is a switching element driving signal. Thus a status in which the actual current value, still exceeds the reference current is generated as the reference timing T4 and the off status of the current flowing through the coils which is started before the reference timing T4 is continued until the actual current value falls below the reference current after the reference timing T4. As a result, there is a case in which the time required for that the current flowing through the coils is turned to off from off status after the current flowing through the coils is turned to on from off status at the reference timing T3 and is turned again to on from off status becomes longer than the period of the reference chopping timing. Thus objectionable acoustic noise is generated.